This invention relates to a fuel injection nozzle for injecting a fuel in two stages; a pilot injection stage and a main injection stage.
In a general fuel injection nozzle, a needle valve and a nozzle spring are received in a hollow main body. The needle valve is urged by the nozzle spring and sat on a main valve seat, thereby closing an injection port. A supply passage is formed in the main body. A high pressure fuel is intermittently and cyclically fed to this supply passage from a fuel injection pump which is driven by an engine. Upon receipt of the fuel pressure, the needle valve is lifted against the nozzle spring and allows the injection port to open. As a consequence, the high pressure fuel is injected through the injection port. When the supply of the high pressure fuel is temporarily stopped, the nozzle valve is caused to sit on the main valve seat under the effect of the nozzle spring. This fuel injection nozzle injects the fuel only once every time the high pressure fuel is supplied.
Recently, in order to improve combustion efficiency, various types of fuel injection nozzles are developed, in which a fuel is injected in two stages; a pilot injection stage and a main injection stage. One type of such a fuel injection nozzle, as disclosed in Japanese Utility Model Publication No. Hei 4-8310, is additionally provided with a dodge plunger. The dodge plunger is received in the main body. A secondary valve seat is formed on a basal end portion of the main body. The secondary valve seat is in connection with the supply passage. The dodge plunger is slidable in an axial direction such that it can sit on the secondary valve seat and it can be lifted from the second valve seat. The nozzle spring is disposed between the needle valve and the dodge plunger and biases them in a direction away from each other.
Before the supply of the high pressure fuel, the needle valve is caused to sit on the main valve seat under the effect of the nozzle spring and the dodge plunger to sit on the secondary valve seat. In the sitting state, a pressure bearing area of the needle valve for receiving a fuel pressure is larger than a pressure bearing area of the dodge plunger for receiving a fuel pressure. For this reason, when the supplied fuel pressure is increased exceeding an initial valve-opening pressure, the needle valve is lifted so that the pilot injection is started and then, a little later, the dodge plunger is lifted. In the lifted state, the pressure bearing surface of the dodge plunger is larger than the pressure bearing area of the lifted needle valve. Accordingly, the biasing force to the dodge plunger caused by the fuel pressure becomes larger than the biasing force to the needle valve caused by the fuel pressure. As a consequence, the dodge plunger contacts the stopper, thereby reaching its full lift position. The nozzle spring is compressed to the extent that the dodge plunger is fully lifted. Therefore, the biasing force of the nozzle spring is correspondingly increased. The needle valve is biased toward the main valve seat and sat thereon by the nozzle spring with such an increased biasing force. In this way, the pilot injection is finished.
After the completion of pilot injection, the needle valve, which is still sitting on the main valve seat, is subjected to the effect of the nozzle spring which is further compressed to the extent of the fully-lifted portion of the dodge plunger. When the fuel pressure is further increased and exceeds the main valve-opening pressure, a lifting force caused by this fuel pressure becomes larger than the force of the nozzle spring to lift the needle valve. As a consequence, the main injection is started. During the time the main injection is undergoing, the dodge plunger is retained by the stopper and held in its lifted position.
When the supply of fuel for a single supply portion is finished and the fuel pressure is decreased, the needle valve having the smaller pressure-bearing surface is sat on the main valve seat and then the dodge plunger is sat on the secondary valve seat. By this, the main injection is finished.
In the above-mentioned conventional fuel injection nozzle, the fuel can be injected in two stages of a pilot injection stage and a main injection stage during the time the engine is revolving at a low speed. However, during the time the engine is revolving at a high speed, the pilot injection and main injection are performed almost in a consecutive manner and cannot be clearly separated. The present inventor analyzed the causes as follows.
As mentioned above, at one stage during the period of the pilot injection, when the dodge plunger is fully lifted by a force caused by the fuel pressure, the nozzle spring is further compressed to bias the needle valve so that the needle valve can sit on the main valve seat. At that time, a delay occurs between the time the dodge plunger is lifted and the time the needle valve is sat on the main valve seat. This delay occurs due to compressive deformation of the nozzle spring . When the engine revolves at a low speed, the increase rate of the fuel pressure is low. Therefore, the delay due to the compressive deformation of the nozzle spring does not become a big problem and the needle valve can sit on the main valve seat. However, the increase rate of the fuel pressure is high when the engine revolves at a high speed. Therefore, the force given to the needle valve caused by the fuel pressure is larger than the force given to the needle valve as the nozzle spring is compressed. As a consequence, the needle valve cannot sit on the main valve seat.
Also, when the engine is revolving at a high speed, the needle valve is unable to keep sitting thereon for a long time but obliged to be lifted almost instantaneously (i.e., in a very short time) from the main valve seat, even if the needle valve is once successfully sat on the main valve seat. The reason is as follows. Since the main valve-opening pressure is determined by a biasing force of the nozzle spring at the time when the dodge plunger is lifted, it is constant irrespective of the number of revolution of the engine. When the engine is revolving at a high speed, the increase rate of the fuel pressure is high and rapidly reaches the main valve-opening pressure.